Swimming of peritrichous bacteria is enabled by an elastohydrodynamic instability

TL;DR

This paper reveals that the swimming ability of peritrichous bacteria like E. coli is facilitated by an elastohydrodynamic instability in their flexible hooks, which prevents destructive force interactions and promotes effective propulsion.

Contribution

It introduces a novel elastohydrodynamic instability mechanism that enables multi-flagellated bacterial swimming, supported by combined computational and theoretical analysis.

Findings

Bacterial swimming is enabled by a bending instability in flexible hooks.

The instability occurs for hooks more flexible than a critical threshold.

Real bacteria are designed to operate near this instability for effective swimming.

Abstract

Peritrichously-flagellated bacteria, such as Escherichia coli, self-propel in fluids by using specialised motors to rotate multiple helical filaments. The rotation of each motor is transmitted to a short flexible segment called the hook which in turn transmits it to a flagellar filament, enabling swimming of the whole cell. Since multiple motors are spatially distributed on the body of the organism, one would expect the propulsive forces from the filaments to push against each other leading to negligible swimming. We use a combination of computations and theory to show that the swimming of multi-flagellated bacteria is enabled by an elastohydrodynamic bending instability occurring for hooks more flexible than a critical threshold. Using past measurements of hook bending stiffness, we demonstrate how the design of real bacteria allows them to be safely on the side of this instability that promotes systematic swimming.